Research Of High-precision Bunch Arrival-time Measurement Based On Electro-optical Modulation Scheme

The rapid development of free electron laser(FEL)is driving the continuous advancement of technology.High-precision bunch arrival-time measurement system provides a diagnostic tool for the sub-10 fs jitter of the FEL facility,provides beambased feedbacks for the FEL facility for an arrival time jitter in the range of 10 fs,and provides sub-10 fs FEL pulse arrival-time measurements in pump-probe experiments.With the continuous development of China's FEL,whether it is based on seeded FEL scheme at SXFEL and DCLS,or based on the SASE scheme at SHINE,higher requirements are imposed on the bunch arrival-time measurement resolution of the facility.Under such opportunities,this dissertation is based on the background of SXFEL,and it has important practical application value to carry out measurement research on high-precision bucnh arrival-time.In this dissertation,we have investigated the research progress of BAM in international laboratories.Compared with the traditional RF BAM scheme,the BAM scheme based on electro-optical intensity modulation scheme can achieve higher arrival-time resolution in long-distance FEL facility,and meet the most stringent measurement accuracy in pump-probe experiment.In order to meet the requirement of SXFEL and SHINE for high-precision bunch arrival-time measurement resolution,the research of high-precision bunch arrival-time measurement based on electro-optical intensity modulation scheme is carried out.Firstly,the composition of BAM subsystems is introduced from the overview of BAM system,and gives a theoretical overview of the time stability of optical synchronization system.The mechanism of Button type and Cavity type pickups which can be used as a candidate for dedicated BAM pickup is presented.We have built a prototype of BAM electro-optical front end.The laser pulse train generated from the synchronization system enters the prototype of BAM electro-optical front end.The laser pulses are divided into two channels by an optical splitter.One of the channels is transmitted to the electro-optical intensity modulator(EOM),and the other is used as a sampling clock to the ADC to synchronize with the laser pulses from the EOM channel.The laser pulses are modulated by EOM which is driven by the radio frequency(RF)signal carrying bunch arrival-time information,and the arrival-time jitter informations in the radio frequency signal are encoded into the amplitude of the laser pulses.The theoretical model of the electro-optical intensity modulator on the laser pulse amplitude is established,and the theoretical formula of the bunch arrivaltime measurement resolution is given.The root mean square stability of 5.5 m K have been reached for a thin Aluminium plate.The experimental verification of the theoretical model of the electro-optical modulation process was carried out.The experimental results and the theoretical model have a good matching,and this have guiding significance for the optimization of the next BAM system version.We have built a prototype of BAM readout electronics.The modulated lasers and clock pulses transmitted from the prototype of BAM electro-optical front-end are photoelectric converted,amplified,attenuated and divided.FPGA complemented the data processing and communication.A method for successful data capture has been proposed.The data bits are precise phase alignment relative to the clock individually.FIFO is used to solve the problem of clock domain crossing,and external trigger signals ars used to intercept the pulse data which contains the arrival-time information of the bunch to filter out the invalid data.A new method based on BAM readout electronics scheme is proposed to test the transmission curve of EOM to determine the static operating point of bias voltage.This solution solves the problem of poor real-time and tedious problems and achieves dynamic correction.Finally,we have tested,evaluated and optimized the performance of BAM system.Detection accuracy is one of the important parameters that can measure the performance of a BAM system.The test results show that the normalized instantaneous pulse amplitude noise detection accuracy of the BAM system is 0.28%,which is close to the laboratories using the same scheme.This lays a good foundation for SXFEL,DCLS and SHINE to enable the measurement resolution of bunch arrival time less than 10 fs.